A polyurethane elastic fiber is a stretchable fiber excellent in an elastic function, and is mixed and knitted or woven with a polyamide fiber, a polyester fiber, cotton, and the like. The resultant fabrics have been widely used in the non-clothing field such as for diapers, bandages, supporters, masks, interior materials of automobiles, nets and tapes as well as in the clothing field such as for foundations, socks, pantyhose, swimwear, sportswear and leotards.
When a polyurethane elastic fiber is used in the field of clothing, the fiber is usually warped and covered, mixed-knitted and mixed-woven, and the resultant fabric is dyed and heat set to give fabric products. When a polyurethane fiber is warped or covered, friction is generated between the fiber and a reed or a guide. Moreover, when a polyurethane elastic fiber is mixed and knitted or woven, friction is generated between the fiber and a guide or a knitting needle. When the friction resistance of the polyurethane elastic fiber is always constant, yarn breakage hardly takes place, and a fabric of high quality having decreased unevenness can be produced. However, actually, yarn breakage caused by a variation in the friction resistance does take place, and unevenness like streaks is occurred to hinder the stability during texturing of the fiber.
In order to improve such texturing stability, imparting a fiber treating agent such as a finish oil to a polyurethane elastic fiber has been commonly done. When a finish oil is imparted in a large amount, the effect of improving the texturing stability is obtained to a certain degree. However, the effect is inadequate. Use of a finish oil in a large amount rather causes a problem of drastic stain on the apparatus, and cannot be said to be economical.
Various investigations on the compositions and adhesion amounts of finish oils have been carried out, and methods of allowing finish oils to contain lubricants such as metallic soaps, silica and silica derivatives have been disclosed (see, for example, Japanese Examined Patent Publication (Kokoku) No. 40-5557, Japanese Unexamined Patent Publication (Kokai) No. 60-239519, Japanese Examined Patent Publication (Kokoku) No. 5-41747, and the like). However, when an insoluble material in a finish oil sticks to a fiber surface, the insoluble material drops off the surface during texturing to cause a problem of fiber scum.
For example, Japanese Examined Patent Publication (Kokoku) No. 58-44767 discloses a method, of lowering the stickiness of a polyurethane elastic fiber, which comprises allowing a polyurethane solution to contain powdery metallic soap in the production step of the polyurethane elastic fiber. However, because the metallic soap is in a dispersed state in the polyurethane solution, the filter and nozzle are clogged to cause a problem of significantly increasing the pressure in the step to impair the step stability.
Furthermore, investigations have also been carried out to improve the texturing stability by modifying the fiber surface, and methods including the following ones have been proposed: a method comprising adding an aliphatic saturated dicarboxylic acid so that the fiber surface is made to have considerable unevenness (Japanese Examined Patent Publication (Kokoku) No. 5-45684); and a method comprising adding barium sulfate having a specific isoelectric point to a polyurethane, and imparting a lubricating finishing agent in combination so that the fiber surface is roughened to have lubricity properties and decreased stickiness (Japanese Patent Publication No. 3279569). However, even these methods cannot make a polyurethane elastic fiber have sufficient texturing stability.